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Neither Phobos nor Deimos have atmospheres. They are too small, with no volatiles to create an atmosphere, and their gravity too low to retain one.
Like Phobos, Deimos has characteristics that make it similar to a C-type asteroid. It is not a spherical moon.
Deimos has a much smoother surface than Phobos, although it does have some craters. The two largest are named Swift and Voltaire, after 19th century writers.
Deimos has a nearly circular orbit around Mars. It may also have been a captured asteroid, but its orbit would have needed a long time to become circular.